There are many subterranean formations containing heavy, i.e. viscous, oils from which the oil cannot be recovered by conventional methods because the oil is too viscous to flow to the production wells without some form of assistance. Such formations are known to exist in the major tar sand deposits of Alberta, Canada and Venezuela with lesser deposits elsewhere, for example, in California, Utah and Texas. The API gravity of the oils in these deposits typically ranges from 10.degree. to 6.degree. in the Athabasca, Alta. deposits to even lower values in the San Miguel sands in Texas, indicating that the oil is of a highly viscous nature.
Various methods have been proposed for recovering the oil in these deposits now that reserves of more easily recovered oils are decreasing, at least in the politically stable areas of the world. These methods include in-situ combustion methods in which part of the oil in the reservoir is burnt by the injection of air or oxygen through an injection well to generate heat so as to reduce the viscosity of the oil and to produce a degree of cracking; the resulting less viscous, cracked oil then acts as a solvent for the heavy oil in place. Solvent recovery techniques have also been proposed, for example in U.S. Pat. Nos. 4,373,585 and 4,293,035, in which a solvent for the heavy oil is injected into the reservoir to form a less viscous solution which can then be recovered by more conventional means. Another technique which has been proposed and used in various forms is steam flooding, in which steam is injected into the formation through an injection well, to heat the formation and, in so doing, to reduce the viscosity of the oil and possibly also, to induce a degree of cracking, resulting in a further reduction in viscosity. Processes of this type can be generally classified as basically of the two well or one well type. In the two well or steam drive type, the steam is injected through an injection well and the injected steam serves to drive the oil towards a separate production well which is located at some horizontal distance (offset) from the injection well. In the one-well or "huff and puff" type operation, a single well is used for both injection and production. The steam is first injected to reduce the viscosity of the oil and to pressurize the formation; after a certain amount of time, steam injection is terminated and the well is turned over to production. A soak period may be allowed to permit the heat to permeate the reservoir to a greater extent before production is initiated in either type of operation. Whether the process is classified as of the one-well or two well types, the well arrangement can, of course, be repeated to cover the field in the manner desired. For example, the two well arrangement may be repeated in regular patterns such as the inverted five spot or inverted seven spot patterns, as described in U.S. Pat. No. 3,927,716. The present method relates basically to the two well type operation, using an injection well or wells and a separate production well or wells at an offset from the injection well.
Horizontally drilled wells have been proposed in various applications, for example, in solvent recovery processes, as described in U.S. Pat. No. 4,385,662 as well as for offshore primary recovery operations as described, for example, in Ocean Industry, June 1984, 35-36 and in certain steam flooding operations mentioned in the Ocean Industry article.
Among the steam flooding operations using horizontal wells are the Kern River, California "huff and puff" project described in the Oil and Gas Journal, 23 August 1982, 51-54, this project also including conventional vertical steam injection wells bisecting the lateral wells. The Cold Lake, Alta. project which used horizontal wells is believed to be of the single well ("huff and puff") type also; the project has been described in Paper No. 79-30-10 of the Petroleum Society of CIM, presented in Banff, Alta. 8-11 May 1979. A similar project was operated at Fort McMurray, Alta, as described in Paper No. 82-33-68 of the Petroleum Society of CIM, presented 6-9 June 1982 in Calgary, Alta., Petroleum Engineer International, September 1982, 40-52. In addition, U.S. Pat. No. 4,248,302 discloses a steam flood recovery method using a highly deviated injection well with production wells situated along the line of the injection well. This proposal has the disadvantages that not only is a relatively large number of production wells required but, in addition, correct positioning of these wells over the injection points is difficult.
Steam flooding processes using horizontal fractures for injecting the steam have been proposed in U.S. Pat. Nos. 3,375,870 and 4,265,310.
U.S. Pat. No. 4,466,485 describes a viscous oil recovery method which employs a steam injection well which extends through the formation in a horizontal direction. The production well is in the conventional vertical position and is completed in the upper two-thirds of the formation. A particular production cycle is employed to maximize recovery but because of the vertical disposition of the production wells, complete drainage of the formation is not assured.